The present disclosure relates to light emitting elements and light emitting devices.
In recent years, GaN-based compound semiconductors have been used for light emitting elements. As a general GaN-based compound semiconductor light emitting element structure, if a single crystal sapphire substrate is used, an n-side semiconductor layer, an active layer, and a p-side semiconductor layer are stacked in that order. Because the sapphire substrate is an insulator, the p-electrode formed on the p-side semiconductor layer and the n-electrode formed on the n-side semiconductor are present on the same side. There are two types of such light emitting elements: a face-up type, in which a light transmissive electrode is used as the p-electrode to extract light from the p-side semiconductor layer side, and a face-down or flip chip type in which a light reflecting film of Ag or the like is used as the p-electrode to extract light from the sapphire substrate side.
In the case of a face-up type light emitting element, a thin metal film such as Ni/Au, a conductive metal oxide film such as ITO, or the like is used for the light transmissive electrode. The surface of the light emitting element is covered by a protective film made of a light transmissive insulating material as described in Japanese Patent Application Publication No. 2010-080542.
In the case of a face-up type light emitting element, light is extracted from not only the upper surface of the semiconductor stack of the light emitting element, but also from the lateral surfaces of the semiconductor stack through the protective film. As disclosed in the above referenced patent document, an oxide such as SiO2, which has good insulating and light transmitting properties, is used for the protective film.
Here, in the case where GaN having approximately 2.4 of reflective index is used for the semiconductor stack, and SiO2 having approximately 1.5 of reflective index is used for the protective film as the suitable material, there is a large difference between the reflective index of the components.
For this reason, an increased percentage of the light traveling externally from the lateral surfaces of the semiconductor stack is reflected at the interfaces between the semiconductor stack and the protective film. The light can be absorbed by pad electrodes or the like while being repeatedly reflected inside the light emitting element, which could reduce the light extraction efficiency.
On the other hand, in the area where the n-pad electrode is disposed near a lateral surface of the semiconductor stack, the light emitted from the lateral surface tends to be relatively easily absorbed by the n-pad electrode. For this reason, allowing the light to be reflected at the interfaces between the semiconductor stack and the protective film in the area near the n-pad electrode so as to be extracted from the upper surface and the lateral surfaces of the other area can better increase the light extraction efficiency. In other words, simply increasing the transmittance of the protective film would not necessarily increase the light extraction efficiency.
Certain embodiments of the present invention aim to increase the light extraction efficiency of a light emitting element and a light emitting device using the light emitting element.